Gas sensor and manufacturing method for the same

ABSTRACT

An air side cover is attached to a proximal end of a housing so as to confine an aerial atmosphere therein. A measured gas side cover is attached to a distal end of the housing so as to confine a measured gas atmosphere therein. A glass sealing material airtightly seals a clearance between an inner surface of an insulator and an outer surface of a sensing element. A contact interface of the glass sealing material protrudes toward a proximal end of the gas sensor compared with at least an adjacent portion of the remainder of the glass sealing material. By melting and hardening a glass pellet, the sensing element is airtightly fixed in the insulator.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a gas sensor installed in anexhaust gas system of an internal combustion engine for a combustioncontrol or else. Furthermore, the present invention relates to a methodfor manufacturing this gas sensor.

[0002] According to a conventional gas sensor, a sensing element isinserted into an insulator. The insulator is fixed in a housing. Ameasured gas side cover is attached to a distal end of the housing. Anair side cover is attached to a proximal end of the housing. Theclearance between the insulator and the housing is airtightly sealed.Similarly, the clearance between the sensing element and the insulatoris airtightly sealed.

[0003] Presence of such an airtight sealing makes it possible toseparate an inside space of the gas sensor into an aerial atmosphere anda measured gas atmosphere.

[0004] In general, the sensing element has a measured gas sensingelectrode exposed to a measured gas and a reference gas sensingelectrode exposed to the air serving as a reference gas. The sensingelement produces a sensing signal representing a gas concentration inthe measured gas based on an ion current or an electric potentialproduced between these electrodes. Hence, measurement of gasconcentration cannot be performed accurately when separation between theaerial atmosphere and the measured gas atmosphere is insufficient.

[0005] Conventionally, powdered material, such as talc, and a sealingglass are layered between the sensing element and the insulator toairtightly separate the aerial atmosphere and the measured gasatmosphere.

[0006] For example, U.S. Pat. No. 5,602,325 discloses a plurality ofsolid-phase sintered glass layers and a plurality of steatite spacerlayers which are alternately stacked in a ceramic sensor holder. Aceramic main body surrounds the alternately stacked glass layers andspacer layers. The ceramic main body extends to an outside housing.Furthermore, thin solid-phase sintered glass layers are interposedbetween the ceramic main body and each spacer layer.

[0007] Furthermore, U.S. Pat. Nos. 5,467,636 and 5,739,414 disclose aglass layer interposed between a first ceramic insulating body and asecond ceramic insulating body. According to this prior art, the glassis subjected to a compressive stress acting in the radial direction(i.e., in the central direction) within an operating temperature zone.

[0008] However, securing airtightness by filling the powdered material,such as talc, requires checking many items to administrate the pressureand the filling amount of the powdered material. This in disadvantageousin costs.

[0009] Furthermore, the glass layer is generally formed through theprocesses of placing the powdered glass material to a predeterminedposition, heating the powdered glass material to melt it, and thencooling the molten glass until it is hardened. This makes it difficultto obtain a highly densified glass sealing material. Accordingly, it isdifficult to maintain satisfactory airtightness for a gas sensor basedon a sealing arrangement using the glass sealing material only.

SUMMARY OF THE INVENTION

[0010] In view of the above-described conventional problems, an objectof the present invention is to provide a gas sensor having anarrangement capable of sealing the clearance between the insulator andthe sensing element with the glass material only. Furthermore, thepresent invention provides a manufacturing method for this sensor.

[0011] In order to accomplish the above and other related objects, thepresent invention provides a gas sensor comprising a cylindricalinsulator, a sensing element airtightly fixed in the insulator, and acylindrical housing having an inside space for placing the insulator,with an air side cover attached to a proximal end of this housing so asto confine an aerial atmosphere therein, wherein a glass sealingmaterial seals a clearance between an inner surface of the insulator andan outer surface of the sensing element, and a proximal end surface ofthe glass sealing material protrudes toward a proximal end of the gassensor at a contact interface of the glass sealing material to the innersurface of the insulator and to the outer surface of the sensing elementcompared with at least an adjacent portion of the remainder.

[0012] The present invention is characterized in that the glass sealingmaterial seals a clearance between the inner surface of the insulatorand the outer surface of the sensing element. The proximal end surfaceof the glass sealing material protrudes toward the proximal end of thegas sensor at the contact interface of the glass sealing material to theinner surface of the insulator and to the outer surface of the sensingelement compared with at least an adjacent portion of the remainder.

[0013] Next, function of the present invention will be explained.

[0014] According to the present invention, the proximal end surface ofthe glass sealing material protrudes toward the proximal end of the gassensor at the contact interface of the glass sealing material to theinner surface of the insulator and to the outer surface of the sensingelement compared with at least an adjacent portion of the remainder(refer to FIG. 2). This arrangement makes it possible to firmly fix theglass sealing material to the sensing element and to the insulator atthe contact interface thereof, thereby maintaining improvedairtightness.

[0015] Accordingly, it becomes possible to surely provide an airtightsealing for the clearance between the sensing element and the insulatorby using a single glass sealing material such as a glass pellet, i.e.,without using powdered material, and without requiring multistagefilling processes of the sealing material, and further without requiringcomplicated check of numerous managing items.

[0016] According to the present invention, it becomes possible toprovide a gas sensor capable of sealing the clearance between theinsulator and the sensing element with the glass material only.

[0017] The glass sealing material is, for example, a material whosecomposition is expressed by B₂O₃—ZnO—SiO₂—Al₂O₃—BaO—MgO.

[0018] This material has an excellent sealing ability for the sensingelement and the insulator. Thus, it becomes possible to ensure thereliable airtight sealing between the glass sealing material and thesensing element as well as between the glass sealing material and theinsulator.

[0019] Furthermore, the present invention is applicable to a gas sensorincorporating a cup-shaped solid electrolytic sensing element as shownin FIG. 1, and also applicable to a gas sensor incorporating amultilayered sensing element.

[0020] Furthermore, the arrangement of the present invention isapplicable to an oxygen sensor and to an air-fuel ratio sensor for anautomotive internal combustion engine. Especially, when formed into amultilayered type, the arrangement of the present invention ispreferably applicable to a NOx sensor, a CO sensor or the like.

[0021] Next, according to the present invention, it is preferable that aprotruding portion of the proximal end surface corresponds to at least98% of the contact interface which extends circumferentially along anentire periphery of the glass sealing material.

[0022] The expression “at least 98% of the contact interface” means thatthe contact interface between the glass sealing material and the innersurface of the insulator and the contact interface between the glasssealing material and the outer surface of the sensing element protrudeby an amount of 98% or more in the circumferential direction.

[0023] When the protruding portion exceeds 98%, it becomes possible tosurely provide an airtight sealing for the clearance between the sensingelement and the insulator by using a single glass sealing material.

[0024] If the protruding portion is less than 98%, gas leakage mayoccur.

[0025] Needless to say, it is most preferable that the proximal endsurface of the glass sealing material protrudes toward the proximal endof the gas sensor entirely along the circumferentially extending contactinterface.

[0026] Next, the present invention provides a method for manufacturing agas sensor comprising a cylindrical insulator, a sensing elementairtightly fixed in the insulator, and a cylindrical housing having aninside space for placing the insulator, with an air side cover attachedto a proximal end of the housing so as to confine an aerial atmospheretherein and a measured gas side cover attached to a distal end of thehousing so as to confine a measured gas atmosphere therein, wherein aglass sealing material seals a clearance between an inner surface of theinsulator and an outer surface of the sensing element, and a proximalend surface of the glass sealing material protrudes toward a proximalend of the gas sensor at a contact interface of the glass sealingmaterial to the inner surface of the insulator and to the outer surfaceof the sensing element compared with at least an adjacent portion of theremainder.

[0027] The method of the present invention comprises the steps ofpreparing a cylindrical glass pellet having an outer shape fitting tothe inner surface of the insulator and having a through-hole into whichthe sensing element is inserted, inserting the glass pellet into theinsulator and placing the sensing element in the through-hole of theglass pellet, and melting the glass pellet and then hardening the moltenglass to firmly seal the clearance between the inner surface of theinsulator and the outer surface of the sensing element.

[0028] According to the manufacturing method of the present invention,the glass pellet configured into a predetermined shape is inserted intothe insulator. Then, the sensing element is disposed in the through-holeof the glass pellet. Thereafter, the glass pellet is melted and hardenedto firmly seal the clearance between the insulator and the sensingelement.

[0029] Accordingly, compared with a conventional method for directlyfilling the clearance with powdered glass material etc. as a glasssealing material, it becomes possible to realize a highly densifiedglass sealing. Accordingly, it becomes easy to obtain a desired sealingin length as well as in volume, thereby realizing a reliable sealing.

[0030] Accordingly, it becomes possible to firmly fix the sensingelement and the insulator at their interfaces so as to maintainexcellent airtightness. Furthermore, it becomes possible to surelyprovide an airtight sealing for the clearance between the sensingelement and the insulator by using a single glass sealing material only.

[0031] As described above, the present invention makes it possible toprovide a manufacturing method for a gas sensor capable of sealing theclearance between the insulator and the sensing element with the glassmaterial only.

[0032] Regarding the shape of the glass pellet, it is possible to formthe glass pellet with side surfaces fitting to the inner surface of theinsulator and to the outer surface of the sensing element. It is alsopossible to configure the glass pellet to have the through-holebeforehand so that the sensing element can be inserted into thisthrough-hole.

[0033] It is also possible to use the glass pellet consisting of aplurality of parts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription which is to be read in conjunction with the accompanyingdrawings, in which:

[0035]FIG. 1 is a vertical cross-sectional diagram showing a gas senorin accordance with a preferred embodiment of the present invention;

[0036]FIG. 2 is an enlarged cross-sectional diagram showing an essentialarrangement of the gas sensor in accordance with the preferredembodiment of the present invention;

[0037]FIG. 3 is a diagram showing the assembling of a sensing element,an insulator, and a glass pellet in accordance with the preferredembodiment of the present invention;

[0038]FIG. 4A is an enlarged cross-sectional diagram showing a contactinterface between an inner surface of the insulator and a glass sealingmaterial in accordance with the preferred embodiment of the presentinvention;

[0039]FIG. 4B is a graph showing a raised amount of the contactinterface between the inner surface of the insulator and the glasssealing material in relation to gas leakage amount in accordance withthe preferred embodiment of the present invention; and

[0040]FIG. 5 is a diagram showing an apparatus measuring the gas leakageamount of a tested gas sensor in accordance with the preferredembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0041] A preferred embodiment of the present invention will be explainedhereinafter with reference to attached drawings. Identical parts aredenoted by the same reference numerals throughout the drawings.

[0042] Hereinafter, a gas sensor according to a preferred embodiment ofthe present invention will be explained with reference to FIGS. 1 to 5.

[0043] In this explanation, a front side of a gas sensor to be exposedto a measured gas is referred to a distal end side and the opposite sideis referred to a proximal end side.

[0044] As shown in FIG. 1, a gas sensor 1 of this embodiment comprises acylindrical insulator 21, a sensing element 15 airtightly fixed in theinsulator 21, and a cylindrical housing 10 having an inside space forplacing the insulator 21. An air side cover 12 is attached to a proximalend of the housing 10 so as to confine an aerial atmosphere 142 therein.A measured gas side cover 13 is attached to a distal end of the housing10 so as to confine a measured gas atmosphere 141 therein.

[0045] As shown in FIG. 2, a glass sealing material 25 airtightly sealsa clearance between an inner surface 210 of the insulator 21 and anouter surface 150 of the sensing element 15. A proximal end surface 255of the glass sealing material 25 protrudes toward a proximal end of thegas sensor 1 at a contact interface 250 of the glass sealing material 25to the inner surface 210 of the insulator 21 and at a contact interface250 of the glass sealing material 25 to the outer surface 150 of thesensing element 15 compared with at least an adjacent portion of theremainder.

[0046] Hereinafter, this embodiment will be explained in more detail.

[0047] The gas sensor 1 of this embodiment is installed in an exhaustsystem of an automotive internal combustion engine and is used for anair-fuel ratio control of the internal combustion engine.

[0048] As shown in FIG. 1, in the gas sensor 1, the measured gas sidecover 13 attached at the distal end of the housing 10 consists of anouter cover 131 and an inner cover 132 cooperatively constituting adouble-layer construction. Both of the outer cover 131 and the innercover 132 are provided with holes 130 through which the measured gas isintroduced into the measured gas side cover 13 so as to form themeasured gas atmosphere 141.

[0049] The air side cover 12 is provided at the proximal end of thehousing 10. An outer cover 121 is overlapped with an outer surface ofthe air side cover 12 at a proximal end thereof via a water-repellentfilter 122. The overlapped portions of the air side cover 12 and theouter cover 121 are provided with holes 120 for introducing air into theair side cover 12 via the water-repellent filter 122.

[0050] The air side cover 12 has a smaller-diameter portion at itsproximal end and a larger-diameter portion at its distal end which areintegrally and continuously formed via a stepped portion 129.

[0051] The air introduced in the air side cover 12 through theair-introducing holes 120 forms the aerial atmosphere 142 of the gassensor 1.

[0052] As shown in FIGS. 1 and 2, the housing 10 is configured into acylindrical shape and has two protrusions 101 and 102 protrudingradially inward from an inner surface thereof.

[0053] The protrusion 101, positioned at the proximal end side, has areceiving surface 103 which supports a tapered portion 211 provided atan outer surface of the insulator 21.

[0054] The insulator 21 is made of alumina ceramic having fineness of98%.

[0055] The tapered portion 211 is supported on the receiving surface 103via an annular metallic packing 11 (refer to FIG. 2). The metallicpacking 11 is made of a nickel member having fineness of 99%.

[0056] The inside space of the gas sensor 1 is airtightly separated intothe aerial atmosphere and the measured gas atmosphere at the portionwhere the metallic packing 11 is disposed.

[0057] An air side insulator 22 is disposed at a proximal end of theinsulator 21. An annular disc spring 220 is disposed between the airside insulator 22 and the stepped portion 129 of the air side cover 12.

[0058] A total of four leads 16 are disposed in an inside space of theair side insulator 22 so as to be electrically conductive with thesensing element 15.

[0059] The sensing element 15, used for detecting an oxygenconcentration, has a multilayer body equipped with a built-in heater.Although not shown in the drawing, the sensing element 15 has two sensorelectrodes for taking out a sensor output signal, two power electrodesfor supplying electric power to the heater, and a total of fourelectrode terminals taken out of the sensor body.

[0060] The four leads 16 are disposed so as to be brought into contactwith these four electrode terminals respectively.

[0061] A proximal end of each lead 16 is connected to a lead 18 via aconnector 17 at an outside of the air side insulator 22. The lead 18extends out of the gas sensor 1 through an elastic insulating member 23disposed at a proximal end side of the air side cover 12.

[0062] As shown in FIG. 2, the sensing element 15 is placed in an insidespace of the insulator 21. The glass sealing material 25 airtightlyseals the clearance between the sensing element 15 and the insulator 21.A proximal end surface 255 of the glass sealing material 25 is raised ata circumferential edge of the glass sealing material 25, i.e., at thecontact interface 250 to the sensing element 15 and to the insulator 21.

[0063] The contact interface 250 is annular. More specifically, theannular contact interface 250 between the glass sealing material 25 andthe insulator 21 has a circular cross section. The annular contactinterface 250 between the glass sealing material 25 and the sensingelement 15 has a polygonal cross section. According to this embodiment,both of the circular contact interface 250 and the polygonal contactinterface 250 are entirely raised along their circumferentialperipheries.

[0064] Furthermore, the glass sealing material 25 contains 21% (weightpercentage) B₂O₃, 34.6% ZnO, 12.2% SiO₂, 4.9% Al₂O₃, 14.2% BaO, and12.7% MgO.

[0065] According to this embodiment, seal fixation between the sensingelement 15 and the insulator 21 of the gas sensor 1 is performed in thefollowing manner.

[0066] As shown in FIG. 3, a cylindrical glass pellet 26 is prepared inaddition to the insulator 21 and the sensing element 15. The cylindricalglass pellet 26 has an outer shape fitting to the inner surface 210 ofthe insulator 21and has a through-hole 260 into which the sensingelement 15 is inserted. A proximal end surface 269 of the glass pellet26 is flat.

[0067] First, the sensing element 15 is inserted into the glass pellet26. Next, the glass pellet 26 is inserted into the insulator 21. Theorder of assembling the sensing element 15, the glass pellet 26, and theinsulator 21 is not limited to the above-described one and therefore canbe inversed.

[0068] Thereafter, these three members are integrally heated in afurnace at the temperature of 800° C. to 950° C. for 30 minutes to fivehours to melt the glass pellet 26 and then naturally cooled down toharden the molten glass.

[0069] When the glass pellet 26 melts, the molten glass can be raised atthe contact interface 250 to the sensing element 15 and to the insulator21 due to surface tension. Therefore, after being naturally cooled down,the glass material protrudes at the contact interface 250 toward theproximal end of the gas sensor 1 while the remainder of the glassmaterial remains substantially flat as shown in FIGS. 1 and 2. Thus, theclearance is airtightly filled with the glass material so as to provideimproved sealing.

[0070] Then, the integrated assembly of the sensing, element 15 and theinsulator 21 is placed in the housing, 10 via a metallic packing 11,thereby constituting the gas sensor 1.

[0071] Next, functions and effects of this embodiment will be explained.

[0072] According to this embodiment, as shown in FIG. 2, the proximalend surface 255 of the glass sealing material 25 protrudes toward theproximal end of the gas sensor 1 at the contact interface 250 of theglass sealing material 25 to the inner surface 210 of the insulator 21and to the outer surface 150 of the sensing element 15 compared with atleast an adjacent portion of the remainder. Thus, the interface betweenthe glass sealing material 25 and the sensing element 15 as well as theinterface between the glass sealing material 25 and the insulator 21 arefirmly fixed so as to provide excellent airtightness.

[0073] Furthermore, according to the method of this embodiment, as shownin FIG. 3, the glass pellet 26 configured into a predetermined shape isinserted into the insulator 21. Then, the sensing element 15 is disposedin the through-hole 260 of the glass pellet 26. Thereafter, the glasspellet 26 is melted and hardened to firmly seal the clearance betweenthe insulator 21 and the sensing element 15.

[0074] Accordingly, compared with a conventional method for filling theclearance with powdered glass material etc. as a glass sealing material,it becomes possible to realize an excellent sealing using the highlydensified glass sealing material 25.

[0075] Accordingly, it becomes possible to surely provide an airtightsealing for the clearance between the sensing element and the insulatorby using a single glass sealing material such as a glass pellet, i.e.,without using powdered material, and without requiring multistagefilling processes of the sealing material, and further without requiringcheck of numerous managing items.

[0076] According to this embodiment, it becomes possible to provide agas sensor capable of sealing the clearance between the insulator andthe sensing element with the glass material only. Furthermore, itbecomes possible to provide a method for manufacturing the gas sensor.

[0077] The following tables 1 to 6 show the components of other glasssealing materials preferable used for the gas sensor in accordance withthe present invention. In each table, the content (wt %) represents avalue expressed in terms of oxide. TABLE 1 component content (wt %) B₂O₃21.0 ± 3 ZnO 34.6 ± 3 SiO₂ 12.6 ± 3 Al₂O₃  4.9 ± 3 BaO 14.2 ± 3 MgO 12.7± 2

[0078] TABLE 2 component content (wt %) B₂O₃ 21.0 ± 3 ZnO 32.0 ± 3 SiO₂19.0 ± 3 BaO 12.0 ± 3 MgO 16.0 ± 3

[0079] TABLE 3 component content (wt %) B₂O₃ 24.0 ± 3 ZnO 45.0 ± 5 SiO₂14.0 ± 3 BaO  7.5 ± 3 MgO  7.5 ± 3

[0080] TABLE 4 component content (wt %) B₂O₃ 24.3 ± 3 ZnO 57.5 ± 5 SiO₂11.0 ± 3 BaO  7.5 ± 3

[0081] TABLE 5 component content (wt %) B₂O₃ 22.6 ± 3 ZnO 34.5 ± 5 SiO₂12.8 ± 3 BaO 11.5 ± 3 MgO 18.6 ± 3

[0082] TABLE 6 component content (wt %) B₂O₃ 19.0 ± 3 ZnO 30.4 ± 5 SiO₂16.0 ± 3 Al₂O₃  5.0 ± 3 BaO 20.0 ± 3 CaO  9.6 ± 3

[0083] To evaluate the present invention, a gas leakage amount wasmeasured in relation to a raised amount of the contact interface 250between the inner surface 210 of the insulator and the glass sealingmaterial 25.

[0084] More specifically, many gas sensors were prepared and classifiedinto two groups according to the condition (i.e., raised or sunkencondition as shown in FIG. 4A) of the contact interface 250 between theinner surface 210 of the insulator 21 and the glass sealing material 25in each gas sensor.

[0085] The gas leakage amount was measured in the following manner.

[0086] Each gas sensor was installed in an apparatus shown in FIG. 5 tomeasure a gas leakage amount at the air side and at the measured gasside. The apparatus shown in FIG. 5 comprises a gas leakage amountmeasuring device 72 equipped with a valve 71 controlling an air supplyamount, a gas sensor attachment jig 74, and a valve 73 provided in apipe connecting the gas leakage amount measuring device 72 and the gassensor attachment jig 74.

[0087] Hereinafter, the measuring method will be explained in moredetail.

[0088] First, the gas sensor 1 is installed in the sensor attachment jig74. The air side and the measured gas side are airtightly separated. Inthis condition, both of the valves 71 and 73 are opened to supply airinto an air reservoir 740 of the attachment jig 74. A rubber packing 741is provided to seal the clearance between the housing 10 of the gassensor 1 and the attachment jig 74.

[0089] If the sealing between the insulator 21 and the glass sealingmember 25 is insufficient, air will leak from the clearance between themas shown by the arrows in the drawing. The pressure in the air reservoir740 decreases with elapsed time.

[0090] Accordingly, this apparatus is used to supply a predeterminedamount of air (4 atm) to the air reservoir 740 and then to measure apressure drop in the air reservoir 740 after the passage of 10 seconds.The gas leakage amount (cm³) can be known from the measured pressuredrop. It is however noted that a preliminary test should be donebeforehand to confirm no presence of gas leakage from other portions.

[0091]FIG. 4B shows the result of measurement.

[0092] From FIG. 4B, it is understood that any gas leakage may occurswhen the raised amount is reduced to 0.

[0093] The raised amount can be precisely measured based on observationof the contact interface 250 on a scanning electron microscopic view.The measurement data of this embodiment are thus obtained through thescanning electron microscopic observation.

[0094] Although this embodiment discloses the measurement result for thecontact interface 250 between the insulator 21 and the glass sealingmaterial 25, similar result was obtained when the gas leakage amount wasmeasured for the contact interface 250 between the sensing element 15and the glass sealing material 25.

[0095] Considering the evaluation test result, it is preferable that aprotruding portion of the proximal end surface 255 of the glass sealingmaterial 25 extends in a circumferential region corresponding to atleast 98% of the contact interface which extends circumferentially alongan entire periphery of the glass sealing material 25.

[0096] This invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof. The presentembodiments as described are therefore intended to be only illustrativeand not restrictive, since the scope of the invention is defined by theappended claims rather than by the description preceding them. Allchanges that fall within the metes and bounds of the claims, orequivalents of such metes and bounds, are therefore intended to beembraced by the claims.

What is claimed is:
 1. A gas sensor comprising a cylindrical insulator,a sensing element airtightly fixed in said insulator, and a cylindricalhousing having an inside space for placing said insulator, with an airside cover attached to a proximal end of said housing so as to confinean aerial atmosphere therein, wherein a glass sealing material seals aclearance between an inner surface of said insulator and an outersurface of said sensing element, and a proximal end surface of saidglass sealing material protrudes toward a proximal end of said gassensor at a contact interface of said glass sealing material to theinner surface of said insulator and to the outer surface of said sensingelement compared with at least an adjacent portion of the remainder. 2.The gas sensor in accordance with claim 1, wherein a protruding portionof said proximal end surface corresponds to at least 98% of said contactinterface which extends circumferentially along an entire periphery ofsaid glass sealing material.
 3. A method for manufacturing a gas sensorcomprising a cylindrical insulator, a sensing element airtightly fixedin said insulator, and a cylindrical housing having an inside space forplacing said insulator, with an air side cover attached to a proximalend of said housing so as to confine an aerial atmosphere therein and ameasured gas side cover attached to a distal end of said housing so asto confine a measured gas atmosphere therein, wherein a glass sealingmaterial seals a clearance between an inner surface of said insulatorand an outer surface of said sensing element, and a proximal end surfaceof said glass sealing material protrudes toward a proximal end of saidgas sensor at a contact interface of said glass sealing material to theinner surface of said insulator and to the outer surface of said sensingelement compared with at least an adjacent portion of the remainder,said method comprising the steps of: preparing a cylindrical glasspellet having an outer shape fitting to said inner surface of saidinsulator and having a through-hole into which said sensing element isinserted, inserting said glass pellet into said insulator and placingsaid sensing element in said through-hole of said glass pellet, andmelting said glass pellet and then hardening the molten glass to firmlyseal the clearance between the inner surface of said insulator and theouter surface of said sensing element.